There's a reason fast-food chains like McDonald's are struggling: they can't rely on the under 18 market in the same way they once could.

A new study reveals that while nearly a third of U.S. kids still eat fast food on any given day, that figure has fallen nearly 20 percent over the span of just a few years.

Researchers found that between 2003 and 2004, nearly 39 percent of kids ate fast-food everyday; between 2009 and 2010, that number had dropped to just under 33 percent. The study, published by the Journal of the American Medical Association, relies on self-reported data from children ages 4 to 19 collected through the National Health and Nutrition Examination Survey.

The segment that has been hit the hardest is not the average burger joint, but the pizza industry. While more than 12 percent of kids in 2003-2004 ordered fast-food pizza on a given day, that number fell to just 6 percent in 2009-2010.

The study also reveals that even when kids are eating fast food, they're consuming fewer calories. For example, while the proportion of kids eating food from a fast-food burger joint on a given day has stayed pretty much the same at around 17 percent, the median calories per meal has dropped nearly 100 calories. In other words, when kids are eating at fast-food joints, they're starting to eat healthier (or at least consuming fewer calories).

Of course, this doesn't mean kids are necessarily eating healthier overall. They could be binging on the increasing number of fast-casual meals with comparable calorie counts that parents feel less guilty feeding them because ingredients are supposedly higher quality (i.e. - choosing Chipotle over Taco Bell). Or, they could be eating homemade meals loaded with calories and sugar.

However, even if the study doesn't definitively determine that children in the U.S. are developing healthier eating habits, it does illuminate an evolving attitude in how kids and their parents approach fast food. Investing in lower calorie options, like serving fruit or milk with kids' meals, is in fast-food restaurants best interest if they want children and parents to continue to spend money at their establishments. In fact, the study was funded by a research grant from McDonald's Corporation to the University of Washington.

Fast-food giants have realized that if they want families to continue to visit, they need to cut calories. However, if the trend against fast food as a daily part of life continues, cutting calories may not be enough to keep fast-food sales consistent with those of decades past.